1. Field of the Invention
The present invention relates to a multi-laser driving apparatus and an adjusting method thereof and further to an image-forming apparatus having the multi-laser driving apparatus.
2. Related Background Art
A laser driving control method in the image-forming apparatus using multiple lasers is a feedback method of monitoring amounts of light emitted from laser diodes, from current values of a photodiode and, if necessary, properly adjusting driving currents of the laser diodes with reference to the light amounts.
In this method, adjustment is necessary for uniformizing relations of amounts of the light emitted from the laser diodes to the monitor currents of the photodiode. In the multiple lasers, the monitor currents have delicate difference or variation between the laser diodes, and thus an independent adjusting circuit is given to each of the laser diodes to carry out an adjustment operation individually.
FIG. 7 is a block diagram to explain a laser driving control circuit according to a prior art example.
Reference symbol 1 designates a multi-beam laser, 1a a laser diode 1 (LD1), 1b a laser diode 2 (LD2), and 1c a photodiode (PD) for monitoring amounts of light from LD1 and LD2. In order to control the multi-beam laser 1, this circuit is comprised of LD1 drive control circuit 51 and LD2 drive control circuit 52.
The operation of the LD1 drive control circuit 51 will be described schematically. In adjustment of LD1, monitor current 1 switch (SW) 8a and initial adjusting resistor 102 are selected to the LD1 drive control circuit 51 from a monitor current select signal 7. The monitor current (Im) outputted from PD passes through a line 4 to be converted to a monitor voltage (Vm) by the initial adjusting resistor 102, LD2 final adjusting resistor 9, and fixed resistor 101. The monitor voltage is amplified by gain amplifier 10 and thereafter is inputted into a comparator 12 to be compared with the reference voltage a of 11. Numeral 13 denotes a sample-hold circuit, which charges a hold capacitor 15 when the reference voltage a 11 is greater than the monitor voltage (Vm) in sampling according to a sample-hold signal 14, but discharges the hold capacitor 15 otherwise. The voltage appearing at the hold capacitor 15 is inputted into a drive amplifier 16 to cause a current set by drive current setting resistor 18 to flow into the collector of drive transistor 17. When a switching transistor 19 is turned on by a laser modulation signal 21, it allows the current from the laser diode 1 of 1a to flow to bring about emission of light. The operation of the LD2 drive control circuit 3 is similar to that of the LD1 drive control circuit 2, and thus description thereof is omitted herein.
In the prior art example described above, the adjustment operation for the multi-beam laser is multiple; for example, in the case of a two-beam laser, the adjustment operation is double that of a single laser. In other words, the adjustment operation has to be carried out for each of single lasers.
Namely, in the case of the structure of the above prior art example, the adjustment had to be carried out in such a manner that for the laser diode 1 the initial adjusting resistor 102 was first adjusted and the final adjusting resistor 9 was adjusted thereafter and that for the laser diode 2 the initial adjusting resistor 105 was first adjusted and the final adjusting resistor 26 was adjusted thereafter.
The present invention has been accomplished in order to solve the above problem and an object of the present invention is to provide a multi-laser driving apparatus and an adjusting method thereof capable of saving the load of the adjusting step, or an image-forming apparatus having such a multi-laser driving apparatus.
A multi-laser driving apparatus according to the present invention is a laser driving apparatus comprising:
a plurality of laser emitting elements;
a light receiving element for monitoring amounts of light from said laser emitting elements;
current-voltage converting means for converting a light amount monitor current outputted from said light receiving element to a light amount monitor voltage; and
control means for controlling a drive current of each of said plurality of laser emitting elements, based on said light amount monitor voltage outputted from said current-voltage converting means,
wherein said current-voltage converting means comprises a common resistor and a plurality of non-common resistors, said common resistor being connected to said light receiving element and to each of said plurality of non-common resistors, and
wherein each of said non-common resistors is connected to said common resistor through switching means and connected to said control means.
A driving method according to the present invention is a laser driving method in which a light receiving element receives light from a laser emitting element, current-voltage converting means converts a monitor current outputted from the light receiving element to a monitor voltage, and control means controls a drive current of the laser emitting element, based on said monitor voltage,
wherein said current-voltage converting means comprises a common resistor and a non-common resistor and said common resistor together with said non-common resistor, is used for converting said monitor current corresponding to emission of light from each of a plurality of said laser emitting elements, to said monitor voltage, and
wherein the non-common resistor together with said common resistor converts said monitor current of a laser emitting element selected from said plurality of laser emitting elements, to said monitor voltage.
An image-forming apparatus according to the present invention comprises the laser driving apparatus described above.